Silicon chips are built on silicon wafers and diced into chips. Known interconnection techniques include wire bonding and so-called flip chip or area array techniques, also known as C4 (controlled collapse chip connection). An array of solder bumps is formed on a chip before the dicing process. The solder bumps may be formed by plating the back side of the wafer through a resist layer. Plating is chemically dependent and can be difficult to control for complex solders. Plating has been used for high melting point solder (e.g., 97% Pb, 3% Sn) and can be used for some lead-free solders desirable for environmental reasons (e.g., SnCu or SnCuAg). However control of exact solder composition for some lead free solders can be difficult or impossible to achieve with good solder composition uniformity and repeatability. Injection molded solder (IMS) is preferred for such materials. A pressure nozzle is employed over the top of a glass plate with holes, which functions as a mold. Solder flows into the cavities and a small amount of solder is left behind, which can be transferred to the wafer. Control of solder composition can be easily controlled using the IMS process.
In current IMS techniques, an etched glass mold is used to form small solder regions, typically bowl-shaped, approximately twice as wide as they are deep. When reflow is initiated, the regions tend to “ball up” (assume a substantially spherical shape) due to surface tension. When transferring material to a workpiece from an IMS mold, wettability, transfer and mold adhesion issues can upset the effective transfer. In addition, if a solder column was desired, the removal of a solder column from the mold would be difficult or impossible depending on aspect ratio of the solder column. To improve the quality of solder transfer and to support novel shape transfer, a mechanism other than good wetting, chemical affinity and capillary action is desired for complete transfer and release from the mold.
Accordingly, it would be desirable to overcome the limitations of prior art approaches.